- equality test on terms before trying convertibility now first tries

- equality test on terms before trying convertibility now first tries
  phyisical equality between terms and then fallback to structural
  equality (same behaviour of ocaml < 3.08)
- changed behaviour of are_convertible. Before this change: equality was
  first tried on terms (as above) then, in case of failure, equality
  between weak head reduced terms was tried. Now weak head reduction is
  always performed. This change is motivated by the convertibility
  between "sin x" and "cos x" (no one was able to see the end of this
  test case). Now this test case completes almost immediately and
  regression test type checking the whole standard library does not
  shown any sensible slow down